1. Field of the invention
The present invention relates to a scanning exposure device for scanning-wise exposure of the image of an original document onto light-sensitive means.
2. Description of the prior art
Scanning exposure devices are well-known in the art of electrophotography and image-reproduction and image-processing. One known form of this kind of device is that used in copiers which have a glass copy board onto which the original document is placed with the image facing downwardly. The copy board is fitted in a carriage which is displaceable over the top of the copier, and a transverse zone of the image is projected through a slotlike opening in the top wall of the copier onto a photoconductor drum by means of an optical projection system. The photoconductor drum is driven in relation to the movement of the copyboard so that the drum, which previously has been uniformly charged, becomes progressively image-wise discharged. The resultant charge image is developed by means of a suitable toner mixture which is then transferred onto a final support, such as a paper sheet, or a transparent foil in the case of the production of transparencies for overhead projection.
Another known form of a scanner is in image processing systems, in which an original document is linewise exposed onto a photosensitive CCD (charge-coupled-device) array in order to produce an electronic line signal which can be digitized and then manipulated for image processing.
While the quality demands are moderate in the field of copiers they are on the contrary high in the fields of digital image processing, and the production of prints or printing plates for the graphic industry, etc.
One feature that plays an important role in the reproduction of an document by means of an optical projection system, is the perfect location of the original document image in the object plane of the projection system. In the case of a scanning exposure wherein the lens system remains stationary, this means that the original document must be displaced bodily during the exposure in a direction which is normal to the scanning line, or the scanning zone. To that end the original document is supported by a glass plate which fits in a carriage which is provided with bearings which allow the displacement of the frame along guide rods or rails.
This known guide construction offers a limited precision which gives acceptable results in the field of copying. Where, however, higher demands are imposed on the precision of the location which is taken by the image-portion which is being reproduced, the precision of the guidance system, the precision of the glass frame, etc. must be high which causes a considerable increase of the costprice of these components.
A scanning device of the described type is disclosed in U.S. Pat. No. 3,737,223.
Another important point is the illumination intensity of the original document which varies as the original document moves out of the object plane. These variations are relatively important since in a scanner the lamps for the illumination are disposed close to the original document, and thus even small variations of the axial position of the original document cause relatively large variations of the illumination strengh. We have found that this phenomenon is particularly disturbing in scanners in digital image processing systems.